The Ultimate Guide to Preparing Subgrade for a Concrete Pour

The most important part of a concrete project happens before the truck even arrives. Professional finishers often say, "A slab is only as good as its base." If your sub-base is poor, your concrete will crack and sink, regardless of how much you spent on the mix or rebar.

Phase 1: Excavation and Organic Removal

You must remove all "overburden"—this includes grass, roots, and topsoil. Organic material decomposes over time, creating air pockets that will cause your slab to drop. You must dig down until you reach "virgin" subsoil that has been compacted by nature for thousands of years.

Phase 2: Proper Drainage and Grading

Water is the enemy of concrete. Your subgrade must be graded so that water flows away from your house or structures. A standard slope is 1/4 inch of drop for every 1 foot of distance. If water pools under your slab, it will soften the soil and cause the concrete to fail.

Phase 3: The Gravel Base

Never pour directly on dirt if you can avoid it. A 4-inch layer of "3/4 inch minus" gravel (crushed stone with dust mixed in) is the ideal base. The dust helps the gravel lock together, while the stone provides the strength. Gravel acts as a "capillary break," stopping moisture from wicking up from the ground into your concrete.

Phase 4: Compaction is Key

You cannot simply rake the gravel flat. You must use a mechanical plate compactor. A properly compacted base should feel like walking on asphalt. If you can see your footprints in the gravel, it's not ready for concrete.

Once your base is rock-solid and perfectly level, use our concrete volume tool to get a final, precise measurement of the area before calling the plant.